dswink & MountainHiker: Thanks for your respective inputs. 'Thought the analogy of stress from catching a cold was an interesting way to think about the body's response. And the bit about planes being pressurized reminded me of an odd concept that crossed my mind many years ago: If one lives high enough (i.e., >8000'), it's possible for that person to have to "descend" in order to fly across the country!

To try to help the OP (I owe him!), I remembered having a link to this web page which can be used to compare atmospheric pressures:

For instance, there is 101.33 kPa at sea level and 57.16 kPa at 15,000' (about 1000' above a 14er) for a difference of -43.6%. Repeating the process for 20,000' (also about 1000' above Cotopaxi) works out to -54.0%. So in round figures, that is about a 2% drop in pressure per thousand feet of elevation gain. (The math is non-linear but that rule-of-thumb should be adequate for the 5000' range of elevations you are considering.)